Measurement of the conductivity of a liquid is often performed in an apparatus for treating a liquid such as liquid chromatograph or desalter. Conventional methods of measuring the liquid conductivity are mainly classified into an electrode method and an electromagnetic induction method.
FIG. 2 is a schematic view for explaining the principle of the two-electrode method. In the drawing, an alternating voltage is applied across electrodes 1, 1 from an alternating power source 2. The conductivity is determined in the two-electrode method by outputting via an operational amplifier 3 the amount of a current flowing across two electrodes 1, 1. It is also known a four-electrode method for measuring the conductivity by determining the voltage fall across two electrodes which are disposed between the electrodes 1 and 1.
Although these two-electrode and four-electrode methods have an advantage that a conductivity ranging from a very low value to a high value (0 to 10,000 .mu.S/cm) can be measured, they have a disadvantage that so-called polarization resistance will not become zero because polarization occurs due to a reverse electromotive force caused by an electrolytic product on the surface of an electrode, a concentration gradient or a decrease in electrode reaction speed even if an alternating current is used.
An electromagnetic conductivity meter using an electro-magnetic induced current as shown in FIG. 3 has been known as second means for measuring the liquid conductivity (refer to Japanese Unexamined Publication Sho 60-190873).
In FIG. 3, a primary coil 6 and a detection coil 7 are wound around first and second excitation rings 4 and 5, respectively. An insulation loop tube 8 passes through the first and second excitation rings 4 and 5 and a liquid to be measured is introduced through the insulation loop tube 8. When an alternating voltage having a given amplitude and a given frequency is applied to the primary coil 6, the liquid in the insulation loop tube 8 serves as one turn coil so that an electromagnetically induced alternating current will flow through the liquid as represented by a dotted line. This causes an alternating electromotive current to be induced in the detection coil 7. The frequency of the induced alternating electromotive current is the same as the frequency of the voltage applied to the primary coil 6 and its amplitude is proportional to the conductivity of the liquid in the insulation loop tube 8. Therefore, the conductivity of the liquid is determined by measuring the electromotive current induced in the detection coil 7.
Although the electro-magnetic conductivity meter shown in FIG. 3 has an advantage that it is excellent in corrosion resistance since it will not cause polarization unlike the above-mentioned electrode method, it has a disadvantage that it can measure only liquids having a high conductivity. In order to enable to measure liquids having a low conductivity, it is necessary to increase the capacity of the coils, the input to the primary coil, the amplification of the detection coil output, the power source capacity and the like. Furthermore, the measuring apparatus will not only become complicated and expensive to manufacture, but also it hard to stably measure the conductivity not higher than 5,000 .mu.S/cm since use of the above mentioned measure is limited in view of noises and other disturbance factors. Since it is necessary to form the tube through which an electromagnetically induced alternating current flows, the liquid flowing path will not only become complicated, but also there is till a serious problem that the shunt flow ratio should be maintained constant.
Therefore, the present invention was made to overcome the disadvantages of the prior art.
It is an object of the present invention to provide an electromagnetic conductivity meter which is simple in structure and is capable of stably measuring the conductivity of liquids having a concentration ranging from low to high and a method of measuring the conductivity of a liquid.